Photographic bath



UNITED STATES PATENT OFFICE.

FELIX A. ELLIOTT, OF ROCHESTER, NEW YORK, ASSIGNOR TO EASTMAN KODAK COMPANY, OF ROCHESTER, NEW YORK, A CORPORATION OF NEW YORK.

rnorooaarmc BATH.

No Drawing.

To all whom it may concern:

Be it known that I, FELIX A. ELLIOTT, a citizen of the United States of America, residing at Rochester, in the county of Monroe and State of New York, have invented certain new and useful Improvements in Photographic Baths, of which the following is a full, clear, and exact specification.

This invention relates to photographic baths and more particularly to acid fixing baths.

It is Well known that solutions of sodium thiosulfate, used for the removal from photographic emulsions of undeveloped silver salts, should be acidified. Alum is generally added to the fixing bath for the purpose ofhardening the gelatine, and if the bath is or becomes alkaline, or too weakly acid, there is a tendency for aluminium compounds to precipitate alumina or aluminium salts. On the other hand, if the bath is too acid, sulfur is precipitated from the sodium thiosulfate. It is necessary, however, to have enough acid present to neutralize the considerable amounts of alkali brought over from the developer.

In an attempt to satisfy these requirements, organic acids are generally used because they are but slightly dissociated in solution and'hence render the bath sufiiciently acid, While since the total amount of alkali which can be neutralized depends on the total amount of acid present, the undissociated acid acts as a reserve from which the acidity, that is the hydrogen ion concentration, of the bath is replenished or maintained with gradually decreasing strength until the acidityOof the bath becomes less than is desirable.

The amounts of these organic acids that can be added, however, are not suiiicient to maintain the acidity of the bath within the desired limits during the life of the bath and it is necessary either to add acid or discard the bath.

I have discovered that the acidity of such a bath may be maintained within'the desired limits for the full life of the bath by the buffer action of certain substances in solution. By the full life of the bath I mean until the sodium thiosulfate is exhausted. By bufi'er action is meant the ability of a solution to resist change in its hydrogen ion concentration through the addition or loss Specification of Letters Patent.

Application filed April 13, 1921.

REISSUED Patented Apr. 4., 1922.

Serial No. 460,995.

of acid or alkali. The unit by which hydrogen ion concentration is measured is defined as represented by the symbol pH or P I have here adopted the former symbol consistently with its use inthe Journal of Biological Chemistry. Full explanation of the theory underlying the buffer action referred to above is contained in the literature, and it is unnecessary here to review it. Reference is made particularly to the comparatively simple explanation found in The Determination of Hydrogen Ions by Clark, published 1920, by the Williams and VVilkins Company, Baltimore, Maryland, and to the extensive bibliography contained therein.

For a neutral solution, the expression pH=7 is approximately correct, andit has been found that the fixing bath should have an acidity or hydrogen ion concentration not 'bufi'ers, that is substances capable by dissociation in accordance with the law of mass action of maintaining the acidity of the fixing bath within the desired limits. The weaker acids, that is, ones which are but partially dissociated in solution, such as the organic acids, for instance acetic acid, citric acid, formic acid and others, have been found to be admirably adapted for use in such a bath. As the buffer material, I have found particularly useful the neutral salts of the alkali metals including ammonium) of the acid used; hus acetic acid and sodium acetate, or citric acid and sodium citrate may be used together with satisfactory results. The buffer material should be slightly acid rather than slightly neutral. Thus sodium dihydrogen phosphate is a suitable buffer, being slightly acid, while disodium hydrogen phosphate is unsuitable. Boric acid can also be used'as a buffer for a stronger acid such as acetic acid.

The following formula is a typical one embodying my invention:

Sodium thiosulfate 300 grams an acid If desired there may be added a suitable hardener such as potash alum, grams.

Another formula is:

Sodium thiosulfate 100 grams Water 600 cc.

to which'may be added a suitable hardener.

While I have endeavored to point out the principles of my invention and to give preferred examples thereof, it is to be understood that there is possible a wide range of equivalents, the use of which I contemplate as within the scope of my invention as de-,

fined in the ap ended claims.

-When in the c aims I refer to the hydrogen ion concentration being maintained at a predetermined strength, do not mean that it is maintained recisely constant dunng the entire life of t e bath, but that it is maintained within certain reasonably narrow limits at a value such that the bath is useful.

Having thus described my invention, what I'claim as new and desire to secure by Letters Patent is:

1. A photographic bath comprising in solution a substance capable of affecting a photographically sensitlve salt, an aci and a substance capable of maintaining the hydrogen ion concentration of the bath at a predetermined strength by dissociation.

2. A photographic bath comprising in solution a substance ca able of affecting a photogra hically sensitlve salt, an acid that is but s ightly dissociated in solution and a substance capable of maintaining the hydrogen ion concentration of the bath at a predetermined strength by dissociation.

3. A photographic bath comprising in solution a solvent for silver halide s,'an acid and a substance capable of maintaining the hydrogen ion concentration of 'the bath at a predetermined strength by dissociation.

4. A photographic bath com rising in solution a solvent for silver hahdes, an acid that is but slightly dissociated in solution, and a substance capable of maintaining the hydrogen ion concentration of the bath at a predetermined'strength by dissociation.

5. A photographic fixing bath comprising solution" of sodium thiosulfate and asubstance capable of maintaining the hydro- 50 by dissociation the hydro v tion of the bath between p 7. A photographic fix ng gen lOIl concentration of the bath at a predetermined strength by dissociation.

6. A photographic fixing bath com rising an acid aqueous solution of sodium t iosulfate and a substance capable of maintaining 11 ion concentra- =4 and pH:6.5. bath comprising tration of the bath an acid aqueous solution of sodium thiosulfate and an acetate.

8. A photographic fixing bat-h comprising an acid aqueous solution of sodium th1osulfate and an acetate, the acetate being capable of maintaining the hydrogen ion concenbetween pH=4 and pH:6.5. I

9. A photographic fixing bath COIDPl'lSlIlg in aqueous solution sodium thiosulfate, an acid and a neutral salt capable of maintaining the hydro 11 ion concentration of the bath at a preditennined strength by dissociation.

10. A photographic fixing bath comprising in solution sodium thiosulfate, an organic acid and a salt of said acid capable of maintaining the hydrogen ion concentration of the bath at a predetermined strength by dissociation.

11. A photogra hic fixing bath comprising in aqueous solution sodium thiosulfate, an organic acid, and a neutral salt of said acid capable of maintaining the hydrogen ion concentration of the bath at a predetermined strength by dissociation.

12. A photographic fixing bath comprising in aqueous so ution sodlum thiosulfate, an acid that is but slightly dissociated in solution, and a substance capable of maintaining the hydrogen ion concentration of the bath at a. predetermined strength by dissociation.

13. A photographic fixing bath comprising in aqueous solution sodium thiosulfate, acetic acid and a substance capable of maintaining the hydrogen ion concentration of the bath at a predetermined strength by dissociation.

14. A photographic fixing bath comprising an acld aqueous solution of sodium thiosu fate and sodium acetate.

15. A photogra hie fixing bath comprismg in aqueous so ution sodium thiosulfate, acetic acid and an acetate.

16. A photogra hic fixin bath comprismg m aqueous so ution so lum thiosulfate, acetic acid and sodium acetate, there being present a sufficient quantity of sodium acetate to maintain bydissociation the hydrogen ion concentration of the bath between pH=4 and pH:.5.

17. A photographic bath with a predetermined hydrogen ion concentration, and containing a substance capable of affecting photographically sensitive salts and a substance capable of maintaining the hydrogen ion concentration within predetermined limits during the life of the bath.

Signed at Rochester, New York, this 7th day of April, 1921.

' FELIX A. ELLIOTT. 

